1. Field of the Invention
The invention relates to a technical field of a constant-temperature type crystal oscillator (hereinafter called constant-temperature type oscillator) using a heating resistor composed of a chip resistor as a heating source, and in particular, to a constant-temperature type oscillator that detects an operational temperature of a crystal unit in real time.
2. Description of the Related Art
Constant-temperature type oscillators keep the operational temperatures of their crystal units constant so as to even the frequency-temperature characteristics of the crystal oscillators. Therefore, the constant-temperature type oscillators are applied to communication facilities (wireless devices) for base stations or the like that are required to have frequency stability, for example, of 0.1 ppm (parts per ten million) or less or in the order of 1 ppb (parts per billion). As one of these, there is a constant-temperature type oscillator configured to use a crystal unit for surface mounting (hereinafter called surface-mounted unit) (see, for example, JP-A-2006-311496).
FIGS. 6A to 7B are diagrams for explanation of one example of a related art constant-temperature type oscillator. FIG. 6A is a cross-sectional view of a related art constant-temperature type oscillator, and FIG. 6B is an outline of a circuit diagram thereof. FIG. 7A is a cross-sectional view of a related art surface-mounted unit of the related art constant-temperature type oscillator, and FIG. 7B is a bottom view thereof.
As show in FIGS. 6A and 6B, the constant-temperature type oscillator has a surface-mounted unit 1, an oscillating part 2a forming an oscillator circuit 2, respective circuit elements 4 forming a temperature control circuit 3, and a circuit substrate 5 on which these are installed. Then, the constant-temperature type oscillator is configured such that the circuit substrate 5 is held with lead wires 8 of a base for oscillator 7 which is made airtight with glass 6, and these are covered with a cover for oscillator 9 by resistance welding or the like. However, there are various forms as an oscillator case composed of the base for oscillator 7 and the cover for oscillator 9, and a form of the oscillator case is selected as needed.
With respect to the surface-mounted unit 1, as shown in FIGS. 7A and 7B, a crystal element 1A is housed in a case main body 10, and the case main body 10 is sealed up with a metal cover 11 by seam welding or the like on a metal ring 10a that is formed on an opening end face of the case main body 10. The case main body 10 is formed to be concave, for example. The case main body 10 is made of laminar ceramic, for example. The case main body 10 has a crystal holding terminal 19 on an inner wall shoulder of the case main body 10. Both sides of one end of the crystal element 1A where a leading electrode (not shown) is extended from an excitation electrode (not shown) are firmly fixed to the crystal holding terminal 19 by an electrically conductive adhesive 20.
Then, the case main body 10 has crystal terminals 12a as mounting terminals 12 electrically connected to the crystal element 1A on a set of diagonal corners of the outer bottom face, for example, and has dummy terminals 12b on the other set of diagonal corners. The dummy terminals 12b are electrically connected to the metal cover 11 via an electrically-conducting path including a through electrode and the like, for example. Normally, the dummy terminals 12b are connected to the ground potential.
Then, the crystal element 1A is formed as an SC-cut crystal element or an AT-cut crystal element, for example. The crystal element 1A has the frequency-temperature characteristic that an extreme value is approximately 80° C. at the higher temperature side higher than or equal to 25° C. as room temperature, and the oscillating frequency varies according to a temperature in any case of both of the SC-cut and AT-cut crystal elements. For example, in an AT-cut crystal element, the frequency-temperature characteristic shows a cubic curve (curve A in FIG. 8), and in an SC-cut crystal element, the frequency-temperature characteristic shows a quadratic curve (curve B in FIG. 8). Incidentally, frequency deviation Δf/f is plotted along the ordinate of the diagram, where f is a frequency at room temperature, and Δf is a frequency difference from the frequency f at room temperature.
As shown in FIG. 6B, the oscillator circuit 2 is formed as a so-called Colpitts type circuit so as to have the oscillating part 2a that forms a resonance circuit along with the surface-mounted unit 1. The oscillating part 2a includes a voltage dividing capacitor, a transistor for oscillation that amplifies and feeds it back, and the like (not shown). Here, the oscillator circuit 2 is formed to be a voltage control type circuit having a voltage-controlled capacitive element 4Cv in an oscillatory loop, for example. In the drawing, Vcc is a power source, Vout is an output, Vgnd is the ground (earth potential), and Vc is a control voltage such as AFC voltage, and these are electrically connected to the circuit substrate 5 via the respective lead wires 8. Further, the power source Vcc is common in the oscillator circuit 2 and the temperature control circuit 3.
In the temperature control circuit 3, a temperature sensing voltage Vt by a temperature sensor (for example, thermistor) 4th and a resistor 4R1 is applied to one input terminal of an operational amplifier 4OA, and a reference voltage Vr by resistors 4R2 and 4R3 is applied to the other input terminal thereof. Then, a differential voltage between the reference voltage Vr and the temperature sensing voltage Vt is applied to the base of a power transistor 4Tr, and electric power from the power source Vcc is supplied to a chip resistor 4h (hereinafter called heating resistor 4h) serving as a heating element. Thereby, the electric power to the heating resistor 4h is controlled with a temperature-dependent resistance value of the temperature sensor 4th, to keep the operational temperature of the surface-mounted unit 1 constant. An operational temperature is to be approximately 80° C. which is a minimum value or a maximum value at room temperature or more (FIG. 8), for example.
A circuit pattern (not shown) is formed on the circuit substrate 5, and the respective circuit elements 4 including the surface-mounted unit 1 are installed on both principal surfaces of the circuit substrate 5. In this example, the heating resistor 4h, the power transistor 4Tr and the temperature sensor 4th in the temperature control circuit 3 and the surface-mounted unit 1 are installed on the bottom face of the circuit substrate 5. The surface-mounted unit 1 is installed on the central area, and the heating resistor 4h, the power transistor 4Tr and the temperature sensor 4th are installed on the outer circumference of the surface-mounted unit 1.
Incidentally, the voltage-controlled capacitive element 4Cv, which is temperature-dependent to greatly vary its characteristic, is installed on the outer circumference of the surface-mounted unit 1. The surface-mounted unit 1 and these circuit elements (4h, 4Tr, 4th, 4Cv) are covered with heat conducting resin 13. Alternatively, the spaces among the surface-mounted unit 1 and these circuit elements (4h, 4Tr, 4th, 4Cv) are filled with the heat conducting resin 13, and these are thermally coupled.
Then, other circuit elements 4 of the oscillator circuit 2 and the temperature control circuit 3 are installed on the top face of the circuit substrate 5. In this case, for example, a capacitor for adjusting oscillating frequency and the like are installed on the top face of the circuit substrate 5, which makes it easy to adjust the oscillating frequency. Then, in particular, the respective circuit elements 4 of the oscillating part 2a having an effect on an oscillating frequency are disposed on the top face of the circuit substrate 5 facing the area covered with the heat conducting resin 13.
Incidentally, U.S. Pat. No. 7,253,694 B2 and US 2007/0268079 A1 each discloses a related art constant-temperature type oscillator.
In the constant-temperature type oscillator having the above-described configuration, the heating resistor 4h and the surface-mounted unit 1 are thermally coupled with the heat conducting resin 13. However, the heat conductivity of the heat conducting resin 13 is less than or equal to 1/100 of that of the electrically-conducting path 14 as a circuit pattern, for example, metal such as gold (Au) or copper (Cu). Thus, the heat conductivity (heat conduction efficiency) between the heating resistor 4h and the surface-mounted unit 1 becomes worse. Meanwhile, the heat conductivity of the heat conducting resin 13, for example, KE-3467 is 2.4 W/(m·K), and those of Au and Cu serving as an electrically-conducting path are 319 W/(m·K) and 403 W/(m·K), respectively.
As shown in FIG. 9, one end sides (power source (Vcc) sides) of the heating resistor 4h and the temperature sensor 4th in the temperature control circuit 3 are directly and electrically connected via the electrically-conducting path 14 on the circuit substrate 5. Therefore, a heated temperature by the heating resistor 4h is directly transferred to the temperature sensor 4th via the electrically-conducting path 14 (for example, metal such as Au) with high heat conductivity, and temperature of the temperature sensor 4th approximately corresponds to the heated temperature by the heating resistor 4h. 
On the other hand, even if the oscillator circuit 2 and the temperature control circuit 3 are connected so as to have the same power source Vcc as their power sources, the heating resistor 4h and the crystal terminal 12a of the surface-mounted unit 1 are electrically connected to one another via the oscillating part 2a. Therefore, a heated temperature by the heating resistor 4h is absorbed by the circuit elements 4 of the oscillating part 2a, and is not directly transferred to the surface-mounted unit 1 (the case main body 10). Thus, an operational temperature of the surface-mounted unit 1 is reduced to be lower than the heated temperature by the heating resistor 4h. In addition, the operational temperature of the surface-mounted unit 1 reaches the heated temperature by the heating resistor 4h after the temperature of the heating resistor 4h reaches the heated temperature.
Therefore, even if a heated temperature by the heating resistor 4h and a detected temperature by the temperature sensor 4th correspond, the detected temperature of the temperature sensor 4th does not correspond with an operational temperature of the surface-mounted unit 1, and a heated temperature higher than the operational temperature of the surface-mounted unit 1 is detected as an operational temperature. Then, an operational temperature of the crystal unit approximates the detected temperature by the temperature sensor 4th as time advances. In this way, specifically the detected temperature by the temperature sensor 4th does not correspond with an operational temperature of the surface-mounted unit 1. Thus, it becomes difficult to control the temperature of the surface-mounted unit 1 in real time for an ambient temperature.
Further, in these constant-temperature type oscillators, the surface-mounted unit 1 is adopted. However, because the other circuit elements 4 are installed as discrete components on the circuit substrate 5, the structure of the constant-temperature type oscillator becomes complicated, and thus simplification thereof becomes difficult.